1. Field of the Invention
The invention pertains to the field of microwave transmitters, and are particularly to high power, solid state, microwave transmitters
2. Description of the Prior Art
Transmitters provide the high power carrier signals in radar and communication systems. These transmitters are modulated in communication systems by signals representative of the information to be transmitted. In pulsed radar systems the transmitters are amplitude modulated to provide the periodic pulse train required for target detection. Some radar systems require continuous wave (CW) transmission for target illumination and tracking. In these systems the transmitter is not amplitude modulated and a CW signal is radiated from the antenna. Each modern radar and communication system requires a transmitter that is stable in both frequency and amplitude and is highly reliable, having long times between failures.
The reliability and stability requirements imposed on modern radar systems precludes the use of vacuum tubes, such as klystron and traveling wave tube (TWT) amplifiers, to amplify the output signal of a low power oscillator to provide the transmitter power. Additionally, these tubes are large devices with large internal structures. The size and geometry of the tubes and their internal structures renders them microphonic and shock sensitive, which increase the noise and spurious signals close to the carrier signal. The noise and spurious signals, which are in the receiver pass band, create serious performance limitations in doppler radar systems. Targets with Doppler shifted frequencies in the receiver pass band will be obscured by the noise and spurious signals and will not be detected.
Many of the disadvantages of the amplifiers may be overcome by the use of solid state amplifiers. These devices have noise figures below 10 dB, compared to noise figures in the order of 30 to 40 dB for tube amplifiers, and are far less susceptible to noise generation caused by vibration and shock. It should be noted, however, that power supply noise and ripple tend to modulate the output of a solid state amplifier. Consequently, the noise and ripple at the output of the solid state amplifier power supplies must be held to a minimum.